This invention relates to a focusing device and more particularly to a system for repositioning a focusing system so as to maintain a scanning beam accurately focused on a videodisc in spite of variations in disc position.
One type of videodisc player utilizes a disc which is rotated on a turntable and which is scanned by a light beam that is accurately focused by a lens onto the videodisc surface. Light reflected from the videodisc is collected by the focusing lens and directed onto a photodetector that drives a video output. In order to read out information recorded at high densities on the disc surface, the scanning beam must be accurately focused on the videodisc surface so that the light spot covers only a very small prescribed area. However, such videodiscs are subject to warpage and surface waviness, which varies the position of the scanned area and some videodisc systems have a variable spacing between disc surface and scanning head. Such variations can be compensated for, by moving the focusing lens toward and away from the turntable, but this requires apparatus for sensing the amount and direction of shifting of the videodisc surface being scanned.